Brightness Gain & Fluoro Artifacts

Brightness gain review 

Brightness gain achieved by minification is equal to the area of the input phosphor to the area of the output phosphor, resulting from the increased electron density and corresponding increased light intensity at the output phosphor 

• Advantages of i.i. over FPD is this brightness gain (vs binning) 

• Flux gain and minification gain are directly proportional

50 to 75 times more electrons than those that entered the input phosphor 5000:1 gain

Flux gain =

Number of output light photons / Number of input x-ray photons

Flux Gain

The # of photons generated at the output phosphor for every photon generated at the input phosphor

The flux gain results from the acceleration of photoelectrons to a higher energy so that they generate more fluoroescent photons at the output phosphor

Minification

The concentration of electrons onto an output phosphor much smaller than the input phosphor

Minification gain

Measured as the ratio of the input phosphor area to the output phosphor are

Concentration of electrons to a smaller area

• analogy of focusing of the sun heat through a magnifying glass causing intense heat on the other side 

Minification Gain =

input phosphor diameter ² / output phosphor diameter ²

Conversion Factor

Radiation intensity incident on the input phosphor

Measured in mR/s 

Approximately 0.01 (1%) times the brightness gain 

Quantity for expressing image intensification 

50 to 300 

Conversion Factor =

Output Phosphor illumination (cd/m²) / Input exposure rate (mR/s)

Importance of Concave Shape of Input Phosphor and Photocathode

Travel distance of electrons must be the same to the output phosphor

Minimizes vacuum stress

(curved - to reach point at same time to form image) 

i.i. Issues

Image lag - blurred images

Burnout 

Blooming - Caused by the input of signals to the video camera that exceed its Dynamic Range 

• Reduce with collimation 

• No Longer a problem in CCD and CMOs Systems 

Saturation (black out) 

Noise 

Image lag in angiography would be …

Detrimental

Blooming 

Saturation would be white on digital subtraction image in IR or Black bone images 

Fluoroscopic Image Noise

The emission of light causing noise

Noise patterns appears to be drifting across the image, creating the appearance of “crawling ants” or “snow”

Can be improved some by increased conversion efficiency in the image intensifier, but since the primary cause is an insufficient flux or exposure rate, the best solution is to raise the mA (tube Current)

Fluoroscopic Image Contrast

Image intensifiers are generally poor at preserving high contrast in the transmitted image 

Some x-rays pass through the input phosphor all the way to the output phosphor, causing it to glow with random fog - like brightness 

Light from the output phosphor can also travel backward to the photocathode, causing it to emit additional electrons in a random pattern 

These effects are destructive to image contrast 

Contrast deteriorates with aging of the intensifier - deterioration can be as high as 10% per year

Pincushion Distortion 

Slight magnification and increased blur around the periphery of the image, caused by projecting the image from a curved input phosphor to a flat output phosphor 

Operation in magnification mode yields a sharper image because the most extreme curvature of the input phosphor at the periphery is eliminated 

From a 25cm diameter to a 10cm diameter, resolution can be increased as much as 50%, from 4 LP/mm

Veiling Glare

Scattered random light from the output phosphor (a haze over the image)

Contributing factors include both scattered electrons passing up through the image intensifier

Degrades apparent image contrast 

Can be tested for by placing a lead disc in the middle of the fluoroscoped image, and checking for any light emission in the central portion of the disc image, which should be black

LOW GLARE =

High Contrast

HIGH GLARE =

Low Contrast

FPD Eliminates …

Veiling glare artifacts

Vignetting

Loss of brightness toward the periphery of the image

A contributing factor is pincushion distortion

Also can occur in optical lenses between the image intensifier and any attached recording device, due to normal scattering of light

Can be tested for by observing a “flat-field” fluoroscoped image, with no object or with a homogeneous object in the beam

Take a spot film of nothing

S Distortion 

Straight Objects appear curved 

Due to an external magnetic field 

DO NOT place i.i. Fluoroscopy rooms in close proximity to your MRI room nor steel support structures 

FPD Issues

Detector cannot acquire another image until all the data is read on current one

Not all of the information/charge is removed all the time… ghosting may occur

Pixeled image due to binning

All dexels have different sensitivity – not consistent

Pixel Gain (Volume averaging)

Unresponsive dels, or pixels

Data line drop off

Loss of spatial resolution with binning